To evaluate the possibility that abrupt withdrawal of propranolol causes over-activity of the sympathetic nervous system or enhancement of platelet function, we examined symp-thetically-mediated heart rate responses, platelet survival and aggregation, and plasma and urine catecholamines in 14 normal subjects before and at various times after propranolol therapy. Preliminary results suggest that propranolol withdrawal may be associated with shortened platelet survival. To date, no consistent change has been observed in heart rate responses to a sympathomimetic drug. Reflex sympathetic cardioaccelerator responses may be augmented, however, after propranolol withdrawal.